A vest, DEFINITELY a vest, especially with my tux, although even with my suits every day I so heavily prefer a three-piece suit. Going all the way back to high school, I've always just liked the fact that I could take the jacket off, if necessary, and still feel kind of properly dressed by virtue of the vest. That's how it feels anyway. And even with the jacket on, the vest just lends a certain feeling of completeness to the formality.